Reliability of resting-state EEG modulation by continuous and intermittent theta burst stimulation of the primary motor cortex: a sham-controlled study

Theta burst stimulation (TBS) is a form of repetitive transcranial magnetic stimulation designed to induce changes of cortical excitability that outlast the period of TBS application. In this study, we explored the effects of continuous TBS (cTBS) and intermittent TBS (iTBS) versus sham TBS stimulation, applied to the left primary motor cortex, on modulation of resting state electroencephalography (rsEEG) power. We first conducted hypothesis-driven region-of-interest (ROI) analyses examining changes in alpha (8–12 Hz) and beta (13–21 Hz) bands over the left and right motor cortex. Additionally, we performed data-driven whole-brain analyses across a wide range of frequencies (1–50 Hz) and all electrodes. Finally, we assessed the reliability of TBS effects across two sessions approximately 1 month apart. None of the protocols produced significant group-level effects in the ROI. Whole-brain analysis revealed that cTBS significantly enhanced relative power between 19 and 43 Hz over multiple sites in both hemispheres. However, these results were not reliable across visits. There were no significant differences between EEG modulation by active and sham TBS protocols. Between-visit reliability of TBS-induced neuromodulatory effects was generally low-to-moderate. We discuss confounding factors and potential approaches for improving the reliability of TBS-induced rsEEG modulation.


Motor evoked potentials (MEPs)
Motor evoked potentials (MEPs) were recorded from the first dorsal interosseous (FDI) muscle of the right hand.In pre-TBS 120 single-pulse TMS (spTMS) were given and corresponding MEPs were recorded.After that, one of three TBS protocols was applied, followed by blocks of 60 spTMS.For the purpose of this study we selected and analyzed only MEPs recorded within the blocks at 5, 20 and 30 min after the TBS.Mean MEP amplitudes for each post-TBS time-point were expressed as the percentage change from pre-TBS.For details of MEP analysis see the previous study [1].

TBS-induced modulation of EEG relative power in the right hemisphere and absolute power in both hemispheres
The results of LMMs of relative power on ROI Right revealed no significant main effects of factor Time in V1 and V2 (all F-values < 3.00, all p-values > 0.05 (corr), partial eta < 0.15, Supplementary Table S1).The results of LMMs of absolute power on ROI Left and Right revealed a single main effect of factor Time in the ROI Left in cTBS V2 in beta 1 band which didn't survive correction for multiple comparison (F-value = 2.76, p-values > 0.05 (corr), partial eta < 0.12, Supplementary Table S2).

Whole-brain analysis of TBS neuromodulatory effects on EEG absolute power
The analysis of absolute power didn't identify any significant clusters.

Modulation of corticospinal excitability
We briefly describe the results of MEP analysis previously published by [1].MEP amplitudes were increased by iTBS and sham TBS in V1 and V2 (Figure S2).cTBS had facilitatory effect on MEPs in V2.For details of results of MEP analysis see the previous study [1].

Comparisons of EEG modulation between visits and test-retest reliability of TBS effects
The results of LMM of relative power in the ROI Right yielded no significant main effect of factor Visit (all Fvalues < 3.00, all p-values > 0.05 (corr.),partial eta < 0.15, Table S4).The results of LMM of absolute power in the ROI Left and ROI Right yielded no significant main effect of factor Visit (all F-values < 2.00, all pvalues > 0.05 (corr.),partial eta < 0.10, Table S5).Overall, reliability of modulation of relative power in ROI Right and absolute power in both ROIs was low-to-moderate (Cronbach's a < 0.75, Table S6).High between-visit reliability of modulation of relative power was obtained in the ROI Right in cTBS at T15 in beta 1 band (Cronbach's a=0.83,Table S6) and in sham TBS also in beta 1 band at T15 (Cronbach's a=0.78,Table S6) and T25 (Cronbach's a=0.98,Table S6).Modulation of absolute EEG power was highly consistent between two visits in the ROI Right after sham TBS in alpha band at T25 (Cronbach's a=0.85,Table S6).

Contingency analysis of individual TBS-induced changes
The contingency analysis revealed that distributions of change types were not statistically different between the visits (χ 2 (3, N=43-45)<7.00, p>0.05 (corr.),see Supplementary table S7).The amount of participants who exhibited significant post-TBS change varied between protocols, visits and frequency bands being on average 76±13% (range 52 and 95%).There was no any clear tendency towards increase or decrease of power attributable to TBS protocol or frequency band.On average 65% of participants exhibited different change types in V1 and V2 (Figure S3).Like the results of analysis of relative power, both active and sham TBS protocols led to large inter-subject variability of types of EEG changes across the visits (cTBS -64±10%, iTBS -79±0%, sham TBS -53±18%, average across alpha and beta1 bands).

Comparison of EEG modulation by active and sham TBS protocols
The results of LMMs of relative power in ROI Right yielded significant main effect of factor Protocol in beta 1 band in V1 which didn't survive correction for multiple comparison (F-value = 4.69, p-values > 0.05 (corr), partial eta = 0.05).The results of LMMs of absolute power in ROI Left and ROI Right also yielded significant main effects of factor Protocol in beta 1 band in V2 which didn't survive correction for multiple comparison (F-value = 4.81, p-values > 0.05 (corr), partial eta = 0.05, and F-value = 3.31, p-values > 0.05 (corr), partial eta = 0.04 correspondingly).

Relationship between EEG spectral power and corticospinal excitability
We performed analysis of relationships between modulation of EEG power in alpha and beta 1 bands in the non-targeted right hemisphere (ROI Right) and corticospinal excitability of the contralateral side by correlating EEG changes and MEPs in those protocols and time points where correlations in the targeted left hemisphere (ROI Left) were found.There was a negative correlation between modulation of relative power in the ROI Right in beta 1 band at T15 and MEP modulation in the left hemisphere at T20 in iTBS V1 (Pearson's r = -0.52,p < 0.05).Modulation of relative power in beta 1 band at T0 in the ROI Right positively correlated with MEP changes in the left hemisphere at T5 in sham V1 (Pearson's r = 0.60, p < 0.05) and in sham V2 (Pearson's r = 0.53, p = 0.01).We also found positive correlation between modulation of absolute EEG power in the ROI Right in alpha band at T15 and MEP modulation at T20 in iTBS V1 (Pearson's r = 0.48, p < 0.05).

Supplementary
Figure S2.Modulation of corticospinal excitability by cTBS, iTBS and sham TBS.Gray round markers -group means of normalized MEP amplitudes (post-TBS/pre-TBS ratios) at 5, 20 and 30 min post-TBS in the initial visit (V1), orange triangles -corresponding values in the retest visit (V2).Error barsstandard error of the mean.Asterisks denote significant MEP changes in comparison to 100% pre-TBS (p < 0.05).Significant MEP facilitation by iTBS in V2 occurred at T10 (not shown).Supplementary figure S3.Conversion of EEG change types in each participant in alpha and beta 1 bands in the initial (V1) and retest (V2) visits.Different types of change are given in different colors: magentaincrease of absolute power in post-TBS in comparison to pre-TBS, bluedecrease of power, green -mixed change including increase and decrease at different post-TBS time points, grey -no change, whitemissing data.

Table S2 .
Supplementary TableS1.Results of linear mixed model analysis of TBS neuromodulatory effects on relative EEG power in ROI Left and ROI Right in the initial (V1) and retest (V2) visits DFdegrees of freedom for the numerator, DF Dendegrees of freedom for the denominator, p-value (uncorr)uncorrected p-value, ROI Left and ROI Rightregions of interest in the left and right hemisphere Results of linear mixed model analysis of TBS neuromodulatory effects on absolute EEG power in the initial (V1) and retest (V2) visits DFdegrees of freedom for the numerator, DF Dendegrees of freedom for the denominator, p-value (uncorr)uncorrected p-value, ROI Left and ROI Rightregion of interest in the left and right hemisphere, bold fontsignificant p-value before correction.Supplementary TableS3Results of linear mixed model and reliability analysis of relative and absolute EEG power at the baseline (pre-TBS) in the ROI Left and Right across six stimulation sessions DFdegrees of freedom for the numerator, DF Dendegrees of freedom for the denominator, p-value (uncorr)uncorrected p-value, ROI Left and ROI Rightregion of interest in the left and right hemisphere Supplementary tableS8.Numerical proportions of types of TBS-induced changes of absolute EEG power in ROI Left in the initial (V1) and retest (V2) visits